An RF ID tag with a battery power supply or of an active type, which is attached to a merchandise article or the like, or carried by a person, transmits an RF signal at a predetermined frequency that carries an ID and other information related to the article or the person, so that the RF signal is received and the information is read out by a reader device. The read-out information is further processed by a computer or the like, so that the distribution of the article or the action of the person is monitored and managed. The active-type RF ID tag with battery power supply has a larger communication range than a passive-type RF ID tag that receives power from a reader/writer device in a contactless manner, and hence is practical in use. However, the active-type RF ID tag transmits an RF signal in a fixed cycle, has a risk of being tracked by a third party, and hence has a problem in the security. To address this security problem, there has been developed an improved active-type RF ID tag that responds only to a tag ID request transmitted by the reader/writer device.
PCT International Publication WO 97/43740 published on Nov. 20, 1997 describes radio frequencies identification device which includes an integrated circuit including a receiver, a transmitter and a microprocessor. The receiver and transmitter together form an active transponder. The integrated circuit is preferably a monolithic single die integrated circuit including the receiver, the transmitter and the microprocessor. Because the device includes an active transponder, instead of a transponder which relies on magnetic coupling for power, the device has a much larger range.
Japanese Patent Application Publication JP 2000-113130-A published on Apr. 21, 2000 describes an IC tag detection system with low power consumption. This system includes a plurality of IC tags provided with different set times of day. Each IC tag includes a communication circuit, a control unit, a power source unit for supplying power from a battery to them, and time measuring means. Each IC tag performs transmission at each prescribed set time of day. This system also includes a detector for detecting the presence or absence of the IC tags based on the communication with them. The detector has a communication circuit, and determines the presence or absence of reception from them successively at the respective set times of day of the respective IC tags. Since the IC tag receives no inquiry from the detector, the IC tag can avoid useless reaction and battery consumption.
Japanese Patent Application Publication JP 2001-251210-A published on Sep. 14, 2001 describes a method of locking a frequency in a transmitter at each of two nodes in a full duplex link, without using a separate reference oscillator in each node. The method provides locking of transmission frequencies of both nodes in a full duplex link at the same time by utilizing information of a received frequency to tune carrier frequencies of the transmitters. The offset of the carrier frequency of the first transmitter is detected as the offset of a second corresponding receiver. The second receiver shifts the carrier frequency of the second transmitter, in response to the detected offset, to inform the first transmitter about the detected offset. The first receiver uses the detected offset to correct the carrier frequency of the first transmitter.
Japanese Patent Application Publication JP HEI 06-290323-A published on Oct. 18, 1994 describes a fare collecting device for toll highway. In the fare collecting device, fare center transmitters for respective lanes are monitored and controlled by a computer to prevent radio waves from radiating simultaneously from a plurality of fare center antennas, to thereby prevent interference of radio waves among the antennas for the lanes.
Japanese Patent Application Publication JP 2004-266550-A published on Sep. 24, 2004 describes an RF tag interrogator system. In the interrogator system, a first interrogator performs transmission at time intervals of 3*t0 resulting from multiplying a prime 3 by a predetermined time t0, and stops the transmission after the transmission for a predetermined period of time. A second interrogator performs transmission at time intervals of 7*t0 resulting from multiplying a prime 7 by a predetermined time t0, and stops the transmission after the transmission for the predetermined period of time. A third interrogator performs transmission at time intervals of 5*t0 resulting from multiplying a prime 5 by the predetermined time t0, and stops the transmission after the transmission for a predetermined period of time. This prevents interference of RF signals from the interrogators as much as possible, and prevents the interference with a certain interrogator communicating with an RF tag for a long period of time.